The present invention relates generally to proton exchange membrane (PEM) electrochemical cells and relates more particularly to a novel PEM electrochemical cell and to a novel PEM electrochemical cell stack.
In certain controlled environments, such as those found in airplanes, submarines and spacecrafts, it is often necessary for oxygen to be furnished in order to provide a habitable environment. An electrolysis cell, which uses electricity to convert water to hydrogen and oxygen, represents one type of device capable of producing quantities of oxygen. One common type of electrolysis cell comprises a proton exchange membrane, an anode positioned along one face of the proton exchange membrane, and a cathode positioned along the other face of the proton exchange membrane. To enhance electrolysis, a catalyst, such as platinum, is typically present both at the interface between the anode and the proton exchange membrane and at the interface between the cathode and the proton exchange membrane. The above-described combination of a proton exchange membrane, an anode, a cathode and associated catalysts is commonly referred to in the art as a membrane electrode assembly.
In use, water is delivered to the anode and an electric potential is applied across the two electrodes, thereby causing the electrolyzed water molecules to be converted into protons, electrons and oxygen atoms. The protons migrate through the proton exchange membrane and are reduced at the cathode to form molecular hydrogen. The oxygen atoms do not traverse the proton exchange membrane and, instead, form molecular oxygen at the anode. (An electrolysis cell, when operated in reverse to generate water and electricity using molecular hydrogen and molecular oxygen as starting materials, is referred to in the art as a fuel cell. Electrolysis cells and fuel cells both constitute electrochemical cells, and all discussion herein pertaining to electrolysis cells is correspondingly applicable to fuel cells.)
Often, a number of electrolysis cells are assembled together in order to meet hydrogen or oxygen production requirements. One common type of assembly is a stack comprising a plurality of stacked electrolysis cells that are electrically connected in series in a bipolar configuration. In a typical stack, each cell includes, in addition to a membrane electrode assembly of the type described above, a pair of multi-layer metal screens, one of said screens being in contact with the outer face of the anode and the other of said screens being in contact with the outer face of the cathode. The screens are used to form the fluid cavities within a cell for the water, hydrogen and oxygen. Each cell additionally includes a pair of polysulfone cell frames, each cell frame peripherally surrounding a screen. The frames are used to peripherally contain the fluids and to conduct the fluids into and out of the screen cavities. Each cell further includes a pair of metal foil separators, one of said separators being positioned against the outer face of the anode screen and the other of said separators being positioned against the outer face of the cathode screen. The separators serve to axially contain the fluids on the active areas of the cell assembly. In addition, the separators and screens together serve to conduct electricity from the anode of one cell to the cathode of its adjacent cell. Plastic gaskets seal the outer faces of the cell frames to the metal separators, the inner faces of the cell frames being sealed to the proton exchange membrane. The cells of the stack are typically compressed between a spring-loaded rigid top end plate and a bottom base plate.
In another typical electrolysis cell stack design, the multi-layer metal screen on the anode side is omitted, and the separator is provided with a set of molded or machined grooves for defining a fluid cavity.
Patents and publications relating to electrolysis cell stacks include the following, all of which are incorporated herein by reference: U.S. Pat. No. 6,057,053, inventor Gibb, issued May 2, 2000; U.S. Pat. No. 5,350,496, inventors Smith et al., issued Sep. 27, 1994; U.S. Pat. No. 5,316,644, inventors Titterington et al., issued May 31, 1994; U.S. Pat. No. 5,009,968, inventors Guthrie et al., issued Apr. 23, 1991; and Coker et al., xe2x80x9cIndustrial and Government Applications of SPE Fuel Cell and Electrolyzers,xe2x80x9d presented at The Case Western Symposium on xe2x80x9cMembranes and Ionic and Electronic Conducting Polymer,xe2x80x9d May 17-19, 1982 (Cleveland, Ohio).
In order to ensure optimal conversion of water to hydrogen and oxygen by each electrolysis cell in a stack, there must be uniform current distribution across the active areas of the electrodes of each cell. Such uniform current distribution requires uniform contact pressure over the active areas of the electrodes. However, uniform contact pressure over the active areas is seldom attained solely through design since the dimensions of the various components of a cell typically vary within some specified limits due to the production methods used in their fabrication. In fact, standard electrolysis cells often show compounded component dimensional variations of about 0.007 to about 0.010 inch due to fabrication limitations, with additional dimensional variations of up to about 0.002 inch due to differential thermal expansion during electrolysis cell operation.
One approach to the aforementioned problem of maintaining uniform contact pressure over the entire active areas of the electrodes has been to provide an electrically-conductive compression pad between adjacent cells in a stack. One type of electrically-conductive compression pad that has received widespread use in the art comprises an elastic disk, said disk being provided with an array of transverse holes and transverse slots. The transverse holes are provided in the disk to allow for lateral expansion during compression of the disk. The transverse slots are provided in the disk so that a plurality of parallel metal strips may be woven from one face of the disk to the opposite face of the disk through the slots.
Other types of electrically-conductive compression pads are disclosed in the following patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,466,354, inventors Leonida et al., issued Nov. 14, 1995; U.S. Pat. No. 5,366,823, inventors Leonida et al., issued Nov. 22, 1994; and U.S. Pat. No. 5,324,565, inventors Leonida et al., issued Jun. 28, 1994.
Compression pads of the type described above comprising an elastic disk having parallel metal strips woven therethroughout are generally capable of compensating for dimensional variations of a cell to maintain uniform contact over the active areas of the cell up to pressures of about 500 psi. However, for many military and commercial applications, the present inventors have noted that it is also important that a cell stack be lightweight and inexpensive. As can readily be appreciated, the above-described compression pad, which is physically separate from the individual cells of a stack, adds weight and expense to the stack and is, therefore, not optimal for such applications. Other components of conventional cells, such as the metal screens, also add weight and expense to the stack.
It is an object of the present invention to provide a novel PEM electrochemical cell.
It is another object of the present invention to provide a PEM electrochemical cell that overcomes at least some of the shortcomings discussed above in connection with existing PEM electrochemical cells.
It is still another object of the present invention to provide a novel PEM electrochemical cell stack.
According to one aspect of the invention, the multi-layer metal screen of a conventional PEM electrochemical cell that is placed in contact with the outer face of the cathode is replaced with an electrically-conductive, spring-like, porous pad. Preferably, said pad is a mat of carbon fibers having a density of about 0.2-0.55 g/cm3, more preferably 0.44-0.55 g/cm3.
Because the pad of the present invention is spring-like (i.e., compressible), a plurality of pad-containing cells can be arranged in a stack, without requiring that a separate compression pad be interposed between adjacent cells for the purpose of maintaining uniform pressure over the active areas of the electrodes, provided that the differential pressure within the stack does not exceed about 400 psi.
In addition, because said pad does not typically experience the same problem of fretting corrosion experienced by a conventional multi-layer screen, a separator plate of the type conventionally used to prevent fretting corrosion of the multi-layer screen may also be omitted from the cell, thereby reducing the cost and weight of the cell.
According to another aspect of the invention, the aforementioned porous pad and the multi-layer anode screen may be approximately equal in thickness to their corresponding peripheral cell frames or, alternatively, the multi-layer anode screen may be thinner than its peripheral cell frame, with the porous pad being thicker than its peripheral cell frame. In the case of the latter alternative, the two straight separators used to axially contain the fluid cavities are replaced with two bent separators shaped to conform to the outer faces of the multi-layer screen and the porous pad, respectively. Preferably, the reduction in thickness to the multi-layer screen is approximately equal to the increase in thickness to the porous pad, and the two bent separators are identical in shape. Moreover, because the porous pad of the present invention obviates the need for a separate compression pad between adjacent cells (for pressure differentials up to about 400 psi), one of the two separators between adjacent cells may be omitted.
Additional objects, features, aspects and advantages of the present invention will be set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.